A ballast that can drive multiple parallel lamps is desirable because the ballast can maintain operation even if one or more of the connected lamps fail. This would reduce the cost of replacing lamps when some lamps fail, particularly in a high-bay lighting area. However, it is difficult to configure a resonant tank for an electronic ballast that can drive multiple parallel lamps without encountering hard-switching, which can damage the inverter switches instantly.
Referring now to FIG. 1, a simple low cost class D series resonant inverter 104 for multiple lamp operation is shown. A direct current (DC) voltage V_rail is supplied, for example, from a power factor correction circuit (not shown) in an electronic ballast. A switching controller 102 (e.g., an integrated circuit) is used to drive a half-bridge inverter formed by a high switch Q1 and a low switch Q2. Switches Q1 and Q2 may be MOSFETs or BJTs. A resonant inductor Lres and resonant capacitor Cres are the major resonant components that form a series resonant tank. A DC blocking capacitor C_dc is connected between the half-bridge inverter and the resonant inductor Lres. A plurality of output capacitors (C2, C3, C4, and C5) limits the lamp currents at certain frequencies. The ballast topology shown in FIG. 1 is inexpensive and reliable, but optimizing the resonant tank to insure multiple lamp operation without encountering hard switching is difficult if not impossible as described with respect to FIG. 2.
FIG. 2 shows resonant tank gain characteristics for one, two, three, and four parallel lamp loads. In FIG. 2, gain is represented by the output current as a function of operation frequency. The resonant tank resonant frequencies for one, two, three and four parallel lamp operation (see FIG. 1) are shown as fres—1, fres—2, fres—3, and fres—4, respectively. The resonant frequencies have a relationship of fres—4<fres—3<fres—2<fres—1. Steady state operation frequency for four parallel lamps is fop, which is between fres—4 and fres—3. Typically, the switching controller 102 reduces the switching frequency from a maximum frequency to a minimum frequency, fop, to start the lamps and maintain a steady state lamp current. During starting, the lamps will be ignited sequentially. As shown in FIG. 2, fop is greater than fres—4, but less than fres—3, fres—2, and fres—4 so that the series resonant inverter will go through capacitive mode load during the starting process. This capacitive mode load will cause hard-switching of the half-bridge inverter and may damage the switches Q1 and Q2. Thus, this simple and low cost topology is unreliable without hard-switching control.